Semiconductor memory devices are widely used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized as either volatile memory, where the stored data is lost if the power source is disconnected or removed or non-volatile, where the stored data is retained even during power interruption. Traditionally, however, non-volatile memory devices occupy a large amount of space and consume large quantities of power, making these devices unsuitable for use in portable devices or as substitutes for frequently-accessed volatile memory devices. On the other hand, volatile memory devices tend to provide greater storage capability and programming options than non-volatile memory devices. Volatile memory devices also generally consume less power than non-volatile devices.
An example of a non-volatile random access memory devices are variable resistance memory devices, which contain many types of resistance change materials, including magnetic materials, doped chalcogenide materials, phase change materials, among others.
One class of variable resistance change materials used in non-volatile random access memory devices are magnetic materials. These devices employ the magneto-resistive effect to store memory states and typically use the magnetization orientation of a layer of magneto-resistive material to represent and to store a binary state. For example, magnetization orientation in one direction may be defined as a logic “0”, while magnetization orientation in another direction may be defined as a logic “1”.
The ability to read stored binary states is a consequence of the magneto-resistive effect. This effect is characterized by a change in resistance of multiple layers of magneto-resistive material, depending on the relative magnetization orientations of the layers. Thus, a magneto-resistive memory cell typically has two magnetic layers that may change orientation with respect to one another. Where the directions of the magnetization vectors point in the same direction, the layers are said to be in a parallel orientation and where the magnetization vectors point in opposite directions, the layers are said to be oriented anti-parallel. In practice, typically one layer, the free or “soft” magnetic layer, is allowed to change orientation, while the other layer, the pinned or “hard” magnetic layer, has a fixed magnetization orientation to provide a reference for the orientation of the free magnetic layer. The magnetization orientation of the two layers may then be detected by determining the relative electrical resistance of the memory cell. If the magnetization orientation of its magnetic layers are substantially parallel, a memory cell is typically in a low resistance state. In contrast, if the magnetization orientation of its magnetic layers is substantially anti-parallel, the memory cell is typically in a high resistance state. Thus, ideally, in typical magneto-resistive memories, binary logic states are stored as binary magnetization orientations in magneto-resistive materials and are read as the binary resistance states of the magneto-resistive memory cells containing the magneto-resistive materials.
Giant magneto-resistive (GMR) and tunneling magneto-resistive (TMR) memory cells are two common types of memory cells that take advantage of this resistance behavior. In GMR cells, the flow of electrons through a conductor situated between a free magnetic layer and a pinned magnetic layer is made to vary, depending on the relative magnetization orientations of the magnetic layers on either side of the conductor. By switching the magnetization orientation of the free magnetic layer, the electron flow through the conductor is altered and the effective resistance of the conductor is changed.
In TMR cells, an electrical barrier layer, rather than a conductor, is situated between a free magnetic layer and a pinned magnetic layer. Electrical charges quantum mechanically tunnel through the barrier layer. Due to the spin dependent nature of the tunneling, the extent of electrical charges passing through the barrier vary with the relative magnetization orientations of the two magnetic layers on either side of the barrier. Thus, the measured resistance of the TMR cell may be switched by switching the magnetization orientation of the free magnetic layer.
Some examples of magneto-resistive memories are disclosed in U.S. Pat. Nos. 7,200,035; 7,196,882; 7,189,583; 7,072,209; 7,038,286; and 6,982,450, assigned to Micron Technology Inc. and incorporated herein by reference.
Another class of variable resistance change materials used in non-volatile random access memory devices are doped chalcogenide materials. Chalcogenides are alloys of Group VI elements of the periodic table, such as Te or Se. In such devices, a fast ion conductor such as a chalcogenide-metal ion and at least two electrodes (e.g., an anode and a cathode) having an electrically conducting material and disposed at the surface of the fast ion conductor are set a distance apart from each other. A specific example of a doped chalcogenide is germanium selenide with silver ions. Typically, to provide the silver ions within the germanium selenide material, germanium selenide is deposited onto the first electrode using chemical vapor deposition. A thin layer of silver is then deposited on the glass, for example by physical vapor deposition or another technique. The layer of silver is then irradiated with ultraviolet radiation. The thin nature of the deposited silver allows the energy to pass through the silver to the silver/glass interface to cause the silver to diffuse into the chalcogenide material. The applied energy and overlying silver result in the silver migrating into the glass layer such that a homogenous distribution of silver throughout the layer is ultimately achieved.
When a voltage is applied to the anode and the cathode, a non-volatile metal dendrite rapidly grows from the cathode along the surface of the fast ion conductor towards the anode. The growth rate of the dendrite is a function of the applied voltage and time; the growth of the dendrite may be stopped by removing the voltage or the dendrite may be retracted back towards the cathode, or even disintegrated, by reversing the voltage polarity at the anode and cathode. Changes in the length and width of the dendrite affect the resistance and capacitance of the variable resistance memory device.
Some examples of variable resistance memory devices and methods of manufacturing such devices are disclosed in U.S. Pat. Nos. 7,149,100; 7,064,970; 6,348,365; and 6,930,909, and U.S. Publication Nos. 2006/0099822; and 2004/0238918, assigned to Micron Technology Inc. and incorporated herein by reference. The memory cells manufactured using the methods disclosed in the above-mentioned publications result in a planar electrode at the top of a layer of chalcogenide material resulting in non-uniform electric field and subsequent signal integrity issues.
In yet another class of variable resistance change materials used in non-volatile random access memory devices are phase change materials. A specific chalcogenide currently used in rewriteable compact discs (“CD-RWs”) is Ge2Sb2Te5. In addition to having valuable optical properties that are utilized in CD-RW discs, Ge2Sb2Te5 also has desirable physical properties as a variable resistance material. Various combinations of Ge, Sb and Te may be used as variable resistance materials and which are herein collectively referred to as GST materials. Specifically, GSTs can change structural phases between an amorphous phase and two crystalline phases. The resistance of the amorphous phase (“a-GST”) and the resistances of the cubic and hexagonal crystalline phases (“c-GST” and “h-GST,” respectively) can differ significantly. The resistance of amorphous GST is greater than the resistances of either cubic GST or hexagonal GST, whose resistances are similar to each other. Thus, in comparing the resistances of the various phases of GST, GST may be considered a two-state material (amorphous GST and crystalline GST), with each state having a different resistance that can be equated with a corresponding binary state. A variable resistance material such as GST whose resistance changes according to its material phase is referred to as a phase change material.
The transition from one GST phase to another occurs in response to temperature changes of the GST material. As shown in FIG. 1, the GST material has a melting temperature Tm and a crystallization temperature Tx. The crystallization temperature Tx is lower than the melting temperature Tm. Both the crystallization temperature Tx and the melting temperature Tm are higher than room temperature. Two curves are shown in FIG. 1, one curve for a-GST and the other for c-GST, h-GST. When the GST material is heated above the melting temperature Tm, the GST material loses its crystalline structure. If the GST material is then rapidly cooled to room temperature, the GST material is formed into an amorphous state—the cooling occurs too fast to allow a crystalline structure to grow. On the other hand, if the GST material is warmed to a temperature above the crystallization temperature Tx, but below the melting temperature Tm, a crystalline structure is able to grow. Once converted into a crystalline structure, the GST material remains in a crystalline structure until it is again heated above the melting temperature Tm. In other words, at room temperature, the GST material is stable in either the amorphous or crystalline phases.
In a phase change memory cell, the heating and cooling can occur by causing differing strengths of current to flow through the GST material. The GST material is placed in a crystalline state by passing a crystallizing current through the GST material, thus warming the GST material to a temperature wherein a crystalline structure may grow. A stronger melting current is used to melt the GST material for subsequent cooling to an amorphous state. As the typical phase change memory cell uses the crystalline state to represent one logic value, e.g., a binary “1,” and the amorphous state to represent another logic value, e.g., a binary “0,” the crystallizing current is referred to as a set current ISET and the melting current is referred to as an erase or reset current IRST. One skilled in the art will understand, however, that the assignment of GST states to binary values may be switched if desired.
The state of the GST material is determined by applying a small read voltage Vr across two electrodes and by measuring the resultant read current Ir. A lower read current Ir corresponds to a higher resistance. Thus, a relatively low read current Ir signifies that the GST material is in an amorphous state and a relatively high read current Ir signifies that the GST material is in a crystalline state.
The phase-changing current is applied to the GST material via electrodes that bound a layer of the GST material. The present manufacturing process results in a planar electrode at the top of the GST layer. Because of the configurations of the bounding surface areas of the two electrodes and the GST layer, current densities within the GST material are not equally distributed.
Accordingly, there is a need for methods and structures where current densities and electric field may be uniformly distributed resulting in subsequent uniform signal integrity.